x/pkgsite: replace all internal Googler links

[mvdan]

I see links like TODO(b/139530312), which I assume is an issue link I can't access or at least some text that doesn't make any sense to me.

If we're going to use this issue tracker alone in the future, like other x repos, we should replace all of those with links to issues here. Some TODOs are this way already, such as TODO(golang/go#38513).

Current counts:

$ git log -1 --oneline
7f80893 (HEAD -> master, origin/master, origin/HEAD) internal/database: fix bug with error handling in transact
$ git grep 'TODO(b/' | wc -l
26
$ git grep 'TODO(.*#' | wc -l
8

/cc @julieqiu @jba

[julieqiu]

Thanks for catching! We'll work on fixing these.

[gopherbot]

Change https://golang.org/cl/238319 mentions this issue: internal: clean up issue links

[gopherbot]

Change https://golang.org/cl/238577 mentions this issue: all: clean up issue links